text: There are two issues here.
  
  Using real world profanity may make books fall afoul of censors. This was even more of an issue back in the 60s and 70s when the first books were published, especially given that minors might take them out of libraries.
  Real world profanity can break suspension of disbelief.  Religious in particular, for Pern, since it is depicted without religion, but even sexual and scatological may make a person think it sounds modern. (I have known readers who were thrown out of historical novels by use of profanity that was actually period, because they didn't realize it was.)